The invention relates to the determination of the state of wear of a drill bit which is used at the lower end of a drill string for drilling a hydrocarbon or geothermal well. In particular, the invention relates to a method of determining the state of wear of the drill bit by detecting and analysing the vibrations produced by the drill bit when drilling.
When drilling a borehole in the earth either in the search for hydrocarbons or for geothermal purposes, a drill string comprising drill pipes, drill collars and a drill bit, is rotated from the surface to drill the wellbore. Roller cone bits are widely used. They have cone shaped steel devices called cones that are free to turn as the bit rotates. Most roller cone bits have three cones although some have two and some have four. Each cone has cutting elements which are circumferential rows of teeth extending from each cone. The cutting elements are either steel teeth which are machined as part of the cone or sintered tungsten carbide teeth which are pressed into holes drilled in the cone surfaces. The geometry of a bit, and more particularly of its cones, is such that when the bit is rotated, the cones rotate, the teeth having a combined rolling and gouging action which drills the formation in contact with the drill bit. As the drilling proceeds, teeth are prone to wear. Exceptionally, they may break. The bearings of the cones are also subject to wear.
The length of time that the drill bit can be used before it becomes excessively worn depends on a variety of factors such as the hardness and composition of the rock and the drill string weight that the operator places on the drill bit. The drill bit should be replaced when its rate of penetration has diminished to an unacceptable level or when torque values in rotating the drill string exceed an acceptable limit. The drill bit must also be replaced in case of a tooth break or a cone bearing failure. In order to avoid the cost of retrieving lost cones from the well bore or to avoid drilling with a worn out bit, the drillers tend to pull out the drill string after a certain drilling period although the drill bit is still working satisfactorily. Pulling out the drill string and tripping it in again with a new drill bit is a time consuming and costly operation. In order to maximise the drilling operation, it would be a significant advantage to obtain information regarding tooth wear and cone or tooth failure. With such information each drill bit could be used to the full extent of its useful life.
As teeth bite against the rock one after another, they generate noise or vibration with frequency components determined by the rate at which teeth successively encounter the rock. Various methods have already been proposed to determine the working state of the bit by recording and analysing the vibrations generated by the drill bit.
It is proposed in U.S. Pat. No. 4,773,263 to obtain the frequency spectrum of the vibrational signal, by processing it through a Fourier transform. The frequency spectrum has been found to include various significant peaks which pertain to different tooth rows of the bit. Peak frequencies tend to increase as teeth wear, because the mean rate of rotation of a cutter (normalized relative to bit speed) tends to increase. Therefore the shift of peak frequencies give useful information on wear and hence whether it is yet time to pull out the drill string. Furthermore, abrupt changes in the form of the frequency spectrum are indicative of abrupt occurrences at the bit such as loss of a tooth. This may lead to the appearance of a new peak as an unbroken tooth is forced to take over the work previously done by the broken tooth. Loss of frequency peaks indicate that a wheel has stuck or is clogged by a ductile rock. However, the success of the method is limited to instances where it is possible to track the position of specific spectral peaks.
In accordance with another method described in U.S. Pat. No. 3,714,822, the extent of wear of the drilling bit is determined by selecting at least one frequency band in the vibration spectrum and by measuring the average amplitude of the signal in the frequency band. In one embodiment, one frequency band is selected centred on the second harmonic of the maximum amplitude frequency and a second band is selected which is centred on the frequency between the second and third harmonic. The ratio between the average amplitudes of the vibrations in these bands is calculated, a ratio of more than two showing that the tool is new and a ratio of about one showing that it is worn.
U.S. Reissue Pat. No. 28,436 relates to a method of determining bit damage by monitoring at the surface the rotary torque of the drill string for characteristic oscillations, the oscillations having a frequency not more than twice and not less than half the expected frequency of torsional oscillations of the drill string. In addition, the monitored oscillations have an amplitude in excess of the amplitude of any oscillations produced by rotation of an undamaged bit.
U.S. Pat. No. 2,985,829 relates to a method of determining the rate of bit rotation by detecting and interpreting vibrations at the surface. By demodulating and filtering the vibration signals, most of the frequency components (and therefore harmonics) different from the bit frequency are eliminated. In addition, the described system is based on the wrong assumption that there is a fixed relation between the speed of a bit and associated roller cones.
The above described techniques have not given full satisfactory results. Therefore a need exists for a method to detect the working conditions of a drill bit, the wear of its teeth or of a cone bearing or a tooth break.